Electrolyte separation wall for the selective transfer of cations through the wall, manufacturing process and transfer process

ABSTRACT

An electrolyte separation wall includes an active layer of a material capable of developing intercalation and deintercalation reactions for the selective transfer of cations through the wall and a support layer made of a porous material acting as support for the active layer. A cation selective transfer process uses such a transfer wall. According to a manufacturing process of such a transfer wall, a solution including an active material in powder form, a binder and a solvent are prepared, then the surface of a support layer made of porous material is coated with the solution and the solvent is evaporated.

FIELD OF THE INVENTION

The invention relates to an electrolyte separation wall for the selective transfer of cations through the wall, a manufacturing process for the said wall and a selective cation transfer process through the said wall. The invention also relates to an electrolytic type process ensuring transfer of cations, through an adapted wall, from a first electrolyte solution containing one or more categories of ions of same charge or of different charges to a second electrolytic solution.

PRIOR TECHNIQUE

A process of this type is already known using as separation wall a wall formed of chalcogenides with molybdenum clusters, notably the Mo₆X₈ phases called Chevrel phases described in the international patent application WO 2009/007598. This process is also the subject of the following publications:

-   -   Electrochemical reactions of reversible intercalation in

Chevrel compounds for cationic transfer—Principle and application on Co²⁺ ion ; S. Seghir, C. Boulanger, S. Diliberto, J. M. Lecuire, M. Potel, O. Merdrignac-Conanec; dans Electrochemistry Communications, 10, 2008, 1505-1508.

-   -   Selective transfer of cations between two electrolytes using the         intercalation properties of Chevrel phases ; S. Seghir, C.         Boulanger, S. Diliberto, M. Potel, J-M. Lecuire; dans         Electrochimica Acta, 55, 2010, 1097-1106.

These documents describe that cations can be transported through the wall made of a material with formula Mo₆X₈ (where X=S, Se, Te) called Chevrel phases where reversible oxidation-reduction systems of following type occur:

Mo₆X₈+xM^(n+)+xne⁻

M_(x)Mo₆X₈

x being a number varying typically between 0 and 4.

These systems are diversified by the nature of the cation M^(n+), of the chalcogene X and of the stoichiometry x of the ternary.

In an experimental setup using the selective transfer process, the transfer wall is placed between two compartments including respectively a platinum-coated titanium electrode which operates as anode and a stainless steel electrode which operates as cathode. The first compartment contains a first electrolyte which contains various cations of an effluent to be treated. The second compartment contains a second electrolyte intended to receive the selected cations.

A direct electrical current is established between the anode and the cathode. In the global electrochemical operation of the set of two compartments, intercalation of the cation occurs at the M_(x)Mo₆S₈/first electrolyte interface (effluent to be treated, mix of cations M^(n+), M′^(n+), M″^(n+) different from each other for example), according to:

Mo₆X₈+xM^(n+)+xne−

M_(x)Mo₆X₈

The deintercalation of this same cation M^(n+) at the M_(x)Mo₆S₈/second electrolyte interface (M^(n+) valorisation solution for example) is done conversely according to:

M_(x)Mo₆X₈

Mo₆X₈+xne⁻+xMn⁺

The mobility of the metallic cation in the Chevrel phase thus allows the transfer of the disolvated cation M^(n+) from one medium to the other without the transfer of any other chemical species from one or the other of the compartments.

A transfer wall in disc form is obtained by hot sintering of a composition powder mix adapted to the stoichiometry of the material required. Thus, discs of active material with a thickness of 2 to 5 millimetres are obtained.

Tests with walls consisting of selenium and sulphur phases have shown that in particular the cations of the following metals can be transferred from one electrolyte to the other: iron, manganese, cobalt, nickel chrome, copper, zinc, cadmium. The current density limits obtained are between 10 and 20 A/m², with Faradaic yields higher than 90%, or even higher than 98%, and a very good selectivity.

The tests also showed that the transfer rate limit increased with the reduction in the thickness of the wall. However, the required mechanical strength of the wall limits the reduction of its thickness.

Moreover, lithium transfer could not be obtained with such walls. Lithium is, however, increasingly demanded in industry, especially for electric car batteries.

TARGETS OF THE INVENTION

The aim of the invention is therefore to provide a selective transfer wall allowing a good transfer rate and with an enlarged choice of transferable cations.

DISCLOSURE OF THE INVENTION

With these targets in mind, the subject of the invention is an electrolyte separation wall including a sealed active layer made of a material capable of developing intercalation and deintercalation reactions for the selective transfer of cations through the wall, characterised in that it includes a support layer consisting of a porous material acting as support for the active layer.

The inventors have succeeded in making a wall with a porous support which provides the mechanical strength and an active layer the thickness of which can be very low. They have observed that the porous support does not obstruct the electrochemical reactions which occur at the level of the active layer. By reducing the thickness of the active layer, the transfer rate achieved is well above the rate limit according to prior art which is one of the targets of the invention.

The porous material is chosen, for example, among mullite, silica, glass fibre, quartz or a ceramic. These materials offer the qualities required to fulfil the role of the wall, that is, the mechanical strength, resistance to the products contained in the electrolytes and porosity.

The porosity of the porous material is for example between 0.4 and 0.6. This value expresses the ratio of material in relation to the volume taken up. It comprises a good trade-off between the volume of the electrolyte present in the porous support and the mechanical strength of the said support.

Particularly, the material of the active layer is a binary or ternary material behaving as a host network with cation reversible accommodation properties according to an oxidation-reduction reaction. The inventors observed that the Chevrel phases are not the only materials which can develop intercalation/deintercalation reactions to form a selective transfer wall but, more generally, host networks which are stable and where oxidation-reduction reactions occur.

The material of the active layer is for example a metallic chalcogenide.

Particularly, the metallic chalcogenide is a chalcogenide with molybdenum clusters (Mo_(n)X_(n+2) or M_(x)Mo_(n)X_(n+2)), X being a chalcogene taken among S (Sulphur), Se (Selenium) and Te (Tellurium), and M being a metal. The number n is chosen for example among 1, 1.5, 2, 3, 4, 5, 6 and 9.

According to another composition, the material of the active layer is a compound of lithium and a metal in oxide form, phosphate or fluoride or a combination of these forms, the metal being chosen among nickel, cobalt, iron, manganese, vanadium, titanium and chrome. It can be seen that these materials are capable of developing intercalation and deintercalation reactions and of transferring cations selectively, especially lithium.

According to a transfer wall manufacturing process, a solution is prepared including an active material in powder form, a binder and a solvent, then the surface of a support layer made of a porous material is coated with the said solution and the solvent is evaporated to form a sealed active layer on the support layer.

It can be seen that the active layer obtained is sealed which guarantees that the electrolytes do not mix when the wall separates them. Also, in spite of the initial powder formulation, the active layer is electrically conductive which shows that the grains are in contact with each other and allow the oxidation-reduction reactions to develop over the complete surface area of the active layer. The layer obtained is very fine in compliance with the target fixed at the start.

The binder is for example poly(vinylidene fluoride).

The solvent is for example 1-methyl-2-pyrrolidone.

The result is already satisfactory when the material in powder form is present in a proportion of 80% in weight excluding the solvent.

The material in powder form has, for example, a grain size between 30 and 100 μm.

According to an improvement, the solution includes, among other things, graphite in powder form. This allows the electrical conductivity of the active layer to be completed.

According to an improvement, the active layer is polished until the support layer appears through the active layer. This thus reduces the thickness of the active layer.

In spite of this reduction, the sealing is preserved and the yield of the wall is not affected. An increase in the limit value of the electrical current density is observed.

The subject of the invention is also a selective cation extraction process by electrochemical transfer characterised in that it uses as electrolyte separation wall a transfer wall as described above and transfer of the cations is ensured through the said transfer wall by generating a potential difference between, on the one hand, the first electrolyte and, on the other hand, the second electrolyte or the said transfer wall to induce an intercalation of the cations in the transfer wall on the side of the first electrolyte, a diffusion of the cations in said wall then their deintercalation in the second electrolyte.

According to other characteristics:

at least one of the electrolytes is non-aqueous. The electrolytes can be different between the compartments, notably by differentiation of the nature of the background salts, by their levels of acidity, by the presence of complexants, by the nature of the solvents notably mineral or organic non-aqueous solvents such as, for example, DMSO, DMF, ionic liquids, solid electrolytes, etc. the transfer wall is electrically connected to a device measuring the potential between the said wall and reference electrodes located respectively in each electrolyte and the potential applied between the said electrolytes is adjusted to suit. the potential difference is generated between the first electrolyte and the transfer wall and the deintercalation of the cations on the side of the second electrolyte is a chemical deintercalation by a chemical oxidising agent in the second electrolyte. a succession of cation transfers is ensured through transfer walls arranged successively between end electrolytes and with one or more intermediary electrolytes between the various transfer walls. the transferred metal is electrodeposited on the cathode. at least two transfer walls of different natures separate the first compartment from respective compartments in parallel for selective transfers of different cations with specific modulated intercalation electrolyses on each of the transfer walls engaged. The transfers to separate compartments allow the specific simultaneous recovery of each of the metals, for example, for a source solution containing Lithium and Cobalt ions with use of an active layer made of Mo₆S₈ for the transfer of the cobalt and a second layer made of LiMn₂O₄ selective transfer of lithium.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood and other features and advantages of the invention will become apparent on reading the following description, the description making reference to the appended drawings where:

FIG. 1 is a cross-sectional view of a transfer wall in compliance with the invention;

FIG. 2 is an X-ray diffraction analysis graph of the porous material for the manufacture of a wall according to FIG. 1;

FIGS. 3 and 4 are schematic views of a test setup to check the porosity or the sealing of the wall of FIG. 1;

FIG. 5 is a schematic diagram of the device,

FIG. 6 shows an arrangement using several compartments and transfer walls in series;

FIG. 7 shows an arrangement using several compartments and transfer walls in parallel.

DETAILED DESCRIPTION

A transfer wall in disc form 2 in compliance with the invention is formed of a porous support 21 on which a fine active layer 22 is deposited. The manufacture of sealed discs is done, in a first phase, by the manufacture of the porous support 21 and, in a second phase, by the application of the active layer 22 to the support 21.

Manufacture of Porous Support

The porous support 21 is commercially available in mullite, quartz or ceramic. As an example, an embodiment is detailed below, which is taken from the protocol given by the article by Garcia-Gabaldon et coll. on the manufacture of ceramic membranes based on kaolin and alumina developing a modulable porosity for their applications as separation membranes in electrochemistry: Effect of porosity on the effective electrical conductivity of different ceramic membranes used as separators in electrochemical reactors, Journal of Membranes Sciences 280 (2006) 536-544.

The protocol is as follows: firstly, a mix intended for 5 g of material consists of:

2.52 g of Kaolin (hydrated aluminium silicate) Al₂Si₂O₅(OH)₄,

3.80 g of Alumina Al₂O₃

1 g of off-the-shelf potato startch.

The powder mix is homogenised in a porcelain mortar then wet with a minimum volume of acetone to prevent aggregates from forming. This mix is dried in free air for 14 h. The powder obtained is then reground manually in the mortar for 10 minutes then by fractions of around 1 g, it is formed into discs by pressing in a 25 mm diameter die under a pressure of 2 tonnes for 5 minutes. The compacted discs are 1 mm thick. The samples are submitted to two successive heat treatment operations.

A first heating operation to 300° C. allows the oxidation in air of the potato starch. This organic binder is eliminated in 1 hour and thus creates the porosity. An additional treatment at 1100° C. for 8 to 24 hours ensures a satisfactory mechanical strength. After this heat treatment, two discs are obtained with a diameter of 24 mm and a thickness of 1 mm. The surface area is 4.5 cm².

An analysis by X-ray diffraction has been done on the porous disc (FIG. 2). The spectrum recorded shows that there are no impurities and the formation of an alumina phase and of a mullite phase.

The evaluation of the porosity of the disc was tested using pH paper and nitric acid HNO₃ in the following manner shown on the diagram of FIG. 3: the change in colour of the pH paper has confirmed the correct porosity of the disc. The porosity checks give average values of 0.553 in volume for initial potato starch contents of 10% and 0.501 for contents of 5%.

Manufacture of Active Layer

The second phase in the manufacture of the disc consists in the physical coating of one face of the porous support 21. In the example shown, the coating is done with a Chevrel phase suspension, formula Mo₆X₈, where X is a chalcogene, in a volatile solvent. The working electrode is prepared from pulverulent compounds Mo₆S₈ or Mo₆Se₈ which comprise the active mass. Added poly(vinylidene fluoride), also called PVDF, plays the role of a binder.

The Mo₆Se₈ phase is obtained from a ceramic synthesis from the Mo^(o) +2MoSe₂ mix homogenised and compressed cold into cylinders at a pressure of 250 MPa, done in sealed molybdenum crucible in arc furnace, under partial argon pressure, then heated for 50 h at 1300° C. The same grinding 50 μm screening treatment is also applied to this compound.

The purity of the synthetic powders is checked by their X-ray diffraction diagrams obtained on a diffractometer.

The synthesis of the sulphur-based Chevrel phase is done by means of a ternary phase with an intermediate metal such as, for example, Cu₃Mo₆S₈. The synthesis of this ternary compound is done in sealed silica tubes in a vacuum at 1000° C. for 50 h. The initial mix is comprised of micrometric powders of Cu, MoS₂ and Mo homogenised in a ball grinder for 30 minutes and compressed cold under a pressure of 250 MPa.

The molybdenum powder is deoxidised under hydrogen current at 1000° C. for 3 h and the MoS₂ powder is prepared in sealed silica tubes by gradually heating the stoichiometric mix of the elements up to 800° C.

The grain size of the pulverulent products engaged is within a range of 30 to 100 micrometres.

The Mo₆Se₈ phase is obtained from a ceramic synthesis from the Mo+2MoSe₂ mix homogenised and compressed cold into cylinders at a pressure of 250 MPa, done in sealed molybdenum crucible in the arc furnace, under partial argon pressure, then heated for 50 h at 1300° C. The purity of the reaction products obtained is checked by their X-ray diffraction diagrams obtained on a diffractometer.

Application of the Active Layer to Porous Support by Coating

Chevrel Phase Matrix Case

A suspension consisting of 95% Chevrel phases in powder form and 5% PVDF is formed in the 1-methyl-2-pyrrolidone, called NMP below, with 0.1 g of the solid Mo₆X₈ phase, 0.005 g of PVDF dispersed in 1 ml of NMP. The whole is stirred for 2 hours.

Using a Pasteur pipette, several drops of the Mo₆X₈ NMP-PVDF suspension is placed on the surface of the porous support disc to cover as uniformly as possible the complete surface area. Then, the whole is placed in the oven for 1 h to eliminate the NMP solvent. Under these conditions, the resulting film of Mo₆S₈ or Mo₆Se₈ adheres to the surface of the disc with thicknesses of around 80 μm. Also, the sealing tests in compliance with FIG. 4 confirm the correct occlusion of the pores of the porous disc with the fact that the pH paper does not change colour. Electrical conductance tests demonstrate good electrical contact between the grains.

Coating techniques using the spin-coating principle have also been used. They give coats of the same configuration as previously.

For the synthesis of the Mo₆S₈ binary phase, copper chemical deintercalation is done by electrochemical means after the coating has been applied.

Li_(x)M_(y)O_(z) Type Oxide Matrix Case

According to another example, the wall is manufactured with as active material a matrix meeting the general Li_(X)M_(Y)O_(Z) formula where y and z are integers, for example, including but not limited to Li_(x)CoO₂, LiMn₂O₄, LiV₃O₈, LiNiO₂ or LiMnO₂. The active material can also include a mix of metals M. The manufacturing principle remains a coating of the porous support by an Li_(X)M_(Y)O_(Z) suspension.

The coating solution is prepared from a pulverulent mix Li_(X)M_(Y)O_(Z) which comprises 80% in weight of the active material, 10% PVDF which plays the role of the binder and 10% carbon which ensures the electrical conductivity. The mix is thoroughly homogenised in a mortar.

A suspension is made in the 1-methyl-2-pyrrolidone with stirring for 2 hours with 0.2 grams of powder mix for 1 ml of NMP.

With the Pasteur pipette, several drops of the Li_(x)M_(y)O_(z) NMP-PVDF solution are placed on the surface of the porous support disc to cover as uniformly as possible the complete surface area. This operation can also be done using a spin-coating technique. Then, the whole is placed in the oven for 1 h to eliminate the NMP solvent. Under these conditions, the resulting oxide film adheres to the surface of the disc with thicknesses of around 80 μm. Also, the sealing tests confirm the correct occlusion of the pores of the porous disc. Electrical conductance tests demonstrate good electrical behaviour of the film.

Whatever the type of matrix, an electrical contact must be placed around the disc using graphite lacquer 23 to follow the interface potentials. The contour of the disc is daubed with the lacquer overlapping onto the face of the active layer 22.

Selective Transfer Process

The diagram on FIG. 5 shows a device for implementing a selective transfer process using transfer walls according to the invention. The device includes a tank 1 including two compartments 11 and 12, adapted to accommodate an electrolyte and separated by a separation wall 13 in which is placed a transfer wall 2 consisting of a disc 2, installed in a sealed manner in the wall 13.

The device also includes an anode A1 placed in the first compartment 11 and a cathode C2 placed in the second compartment 12. A potential difference AE can be applied between the anode Al and the cathode C2 by means known themselves to impose and check a current i between the electrolytes E1 and E2.

The active layer 22 is placed on the side of the first compartment 11, even if the system also operates when it is placed on the side of the second compartment 12. A spring-mounted mobile contact system 44 ensures an electrical connection with the contour of the disc 2 covered by graphite lacquer and allows the disc to be connected to a control device, adapted notably to measure the interface potential Ei1, Ei2 of the disc in relation to the reference electrodes 33, 34 placed respectively in each compartment 11, 12 of tank 1, as shown on FIG. 5.

Typically, the device is used as follows:

The compartments 11 and 12 are filled with the required electrolyte, for example, and in an in no way limitative manner, 100 ml of 0.5 M Na₂SO₄+M_((i))SO₄ as first electrolyte E1 in the first compartment 11, and 100 ml of 0.5 M Na₂SO₄ as second electrolyte E2 in the second compartment 12, with M_((i)) being one or more metallic cations that are to be separated. The anode A1 is placed in the first compartment 11 and the cathode C2 in the second compartment 12, and the contact 44 of the disc is connected with the potentiometric control means, connected to the reference electrodes 33, 34 immersed in the electrolytes E1 and E2. The interface potentials can thus be checked and adjusted to suit the global potential ΔE applied between the anode A1 and the cathode C2, to obtain a current density related to the operational surface area of the transfer wall 2, or of all of the transfer walls arranged in parallel included, for example, between 2 and 200 A/m².

A global intensiostatic state is established between the anode A1 and the cathode C2. Let us call RH, for host network, the material of the active layer 22. In the global electrochemical operation of the two compartments, the electrolyte E1 being an original solution to be treated including a mix of cations of different metals and of identical or different charges, M^(n+), M′^(n+), M″^(n′+) for example, and the electrolyte E2 being a valorisation solution of metal M, the following occurs:

the intercalation of the cation M^(n+) at the interface of the active layer 22 with the electrolyte E1, as follows:

RH+xM^(n+)+xne⁻

^(M) _(x)RH

the deintercalation of this same cation at the interface of the active layer 22 with the electrolyte E2 (valorisation solution of M^(n+) for example), which is done vice versa as follows:

M_(x)

RH+xne⁻+xM^(n+)

The mobility of the metallic cation in the host network thus allows the transfer of the disolvated cation M^(n+) from one medium to the other without transfer of any other chemical species from one or the other of the compartments.

Also note, in a general manner, that the electrolytes placed in the two compartments 11, 12, including the anode A1 and the cathode C2, can be different, notably by the nature of the background salts, by the level of acidity, by the presence of complexants, by the nature of the solvents, notably organic or mineral non-aqueous solvents (DMSO, DMF, ionic liquids, solid electrolytes, etc.). It is thus possible, for example, to do an ionic transfer from a sulphate medium to a chloride medium without scattering of the said medium.

In the variant on FIG. 6, the tank includes three compartments. The two end compartments 11′, 12′ are equivalent to compartments 11 and 12 of the example shown on FIG. 1. An additional compartment 15, containing an electrolyte E3, is located between the two compartments 11′ and 12′ and separated from these by separation walls 13′, 13″ each including one or more transfer walls 2′, 2″ according to the invention. These transfer walls 2′, 2″ can be of same nature, to simply increase the selectivity of the transfer from the compartment 11′ to the compartment 12′. They can also be of different natures and be managed differently by a specific check of the potentials applied between the various compartments for example to do a separation of different cations. For example, two types of cations can be transferred from the compartment 11′ to the compartment 15, and only one from the compartment 15 to the compartment 12′. Various combinations of discs and transfer parameters can thus be used to do the various required separations and treatments.

In the case of the variant on FIG. 6, the intermediary electrolyte or electrolytes E3 can also be identical to or different from one or the two electrolytes E1 or E2.

In another variant, shown on FIG. 7, the tank includes three compartments. The central compartment 11″ is equivalent to the first compartment 11 of the example shown on FIG. 2. The LH compartment 12″ is equivalent to the second compartment 12 of the example shown on FIG. 2. An additional compartment 16, containing an electrolyte E3 is located to the right of the first compartment and separated from it by a separation wall 13′″ including one or more transfer walls 2′″ according to the invention. These transfer walls 2′″ are of different natures according to the separation wall 13, 13′″, to selectively transfer a specific cation for each compartment. For example, the first electrolyte El is a source solution containing cobalt and lithium ions. The first transfer wall 2, between the first and the second compartments 11″, 12″, has an active material made of Mo₆S₈ for the selective transfer of the cobalt, whereas the second transfer wall 2′″, between the first and the third compartments 11″, 16, has an active material made of LiMn₂O₄ for the simultaneous selective transfer of lithium. The first compartment 11″ includes an anode A1″ in a manner to create a transfer current between the said anode A1″ and a cathode C2″ in the second compartment, and another transfer current between the said anode A1″ and a cathode C3 in the third compartment 16.

Example 1

A porous disc 21 covered by a sulphur Chevrel phase based active layer 22 is used as transfer wall 2, as described previously, in a setup in compliance with FIG. 5. The transfer of cations between the first compartment 11, containing the first electrolyte E1 (cation solution 0.1 M M²⁺ in medium 0.1 M Na₂SO₄, 0.1 M H₂SO₄) and the second compartment 12 containing the second valorisation electrolyte E2 with 0.1 M Na₂SO₄ and 0.1 M H₂SO₄ has been studied for the two types of discs based respectively on Mo₆S₈ and Mo₆Se₈, and at different current densities. The aim of the study was to check, for different cations M^(n+), the faradaic yields of the transfers, to determine the limit conditions for the porous Mo₆S₈ and Mo₆Se₈ discs and to evaluate the current density limit. The transfer process is based on a preliminary conditioning with a quantity of inserted cations estimated from the Mo₆X₈ mass deposited for a stoichoimetry of M_(x)/₂Mo₆X₈. Tables 1 and 2 show the results obtained for an active material respectively Mo₆S₈ and Mo₆Se₈.

TABLE 1 Mo₆S₈ Metal M Co Ni Cd Zn Mn In Transfer faradaic 98 90 98 96 92 No yield (%) transfer Limit current 70 70 70 70 70 No density (A/m²) transfer

TABLE 2 Mo₆Se₈ Metal M Co Ni Cd Zn Mn In Transfer faradaic No No 99 97 94 97 yield (%) transfer transfer Limit current No No 70 70 70 70 density (A/m²) transfer transfer

It can be seen that the faradaic yields are interesting as they are above 90% and that the current densities are well above those of prior art (around 16 A/m²).

For the sulphur phase, the quantitative transfer is established at rates between 2.10⁻² mol/h/m² at low current density (3.2 A/m ²) and 3 mol/h/m² at high current density (70 A/m²). The latter value appears to be a limit rate for Mo₆S₈ junction thicknesses estimated at 80 μm. Concerning the disc with active material Mo₆Se₈, the transfer rates in these selenium phases are located at the same order of magnitude as for the sulphur phase, that is 5.10⁻² mol/h/m² for 3.2 A/m² and 4 mol/h/m² for 70 A/m² only for cations Cd²⁺, Zn²⁺, Mn²⁺, Cu²⁺, and In³⁺. Tables 3 and 4 give the transfer rates for a current density of 70 A/m² for each element.

TABLE 3 Mo₆S₈ Metal M Co²⁺ Ni²⁺ Fe²⁺ Cd²⁺ Zn²⁺ Mn²⁺ Cu²⁺ In³⁺ Transfer 1.32 1.11 1.65 3.07 1.43 3.89 4.15 0 rate (mol/h/m²)

TABLE 4 Mo₆Se₈ Metal M Co²⁺ Ni²⁺ Fe²⁺ Cd²⁺ Zn²⁺ Mn²⁺ Cu²⁺ In³⁺ Transfer 0 0 0 3.43 4.17 3.23 2.08 4.08 rate (mol/h/m²)

Selectivity

In the cation selective transfer process, the first electrolyte E1 contains a mix of cations only one type of which is transferred through the wall. The selectivity is induced by the fact that during the electrolysis operation, the voltage applied between the two faces of the active layer 22 allows the intercalation and the deintercalation of only one type of cations. To transfer the other cations, a higher potential must be applied, which is not the aim of the process. The type of cation which is transferred has a minimum intercalation potential and a maximum deintercalation potential which are expressed in relation to the reference potential given by a saturated calomel electrode (SCE).

Transfer experimentations have been done for synthetic mixes of cations such as: Co/Ni, Cd/Zn, Cd/Ni, Zn/Mn, Cd/Co, Co/Fe, Ni/Fe and Cd/Co/Ni.

In the following examples, done from two equimolar cation mixtures (0.1 M), the transfer selectivity is expressed by a transfer selectivity rate of cation M^(n+) represented by the ratio M_(t) ^(n+)/ΣM_(it) ^(n+) of the quantity of cations transferred M_(t) ^(n+) for the species considered with the sum of cations transferred of all species M_(it) ^(n+) in the compartment 2, for example Co_(t)/(Co_(t)+Ni_(t)) for the Co²⁺ +Ni²⁺ mix.

This ratio therefore approaches 100% as selectivity increases and takes a value of 50% if no selectivity develops.

Tables 5 and 6 give a summary of the selectivity rates obtained for the various mixes with different current densities. The values given for the different current densities correspond to the average of the selectivity rates obtained at each hour during the electrolysis of 1 to 7 hours.

TABLE 5 Active material of transfer wall: Mo₆S₈ Minimum Maximum Current Current Mixes intercalation deintercalation density density Cation Other potential potential 3.2 A/m² 70 A/m² transferred cations (mV/SCE) (mV/SCE) (%) (%) Cd Zn −592 −327 93 93 Co Ni −624 −250 99 99 Cd Ni −582 −278 70 78 Zn Mn −814 −196 57 60 Cd Co −585 −373 47 89 Co Fe −751 −175 60 59 Ni Fe −756 −118 49 53 Cd Co/Ni −660 −250 63 77 In Cd/Zn / / / /

TABLE 6 Active material of transfer wall: Mo₆Se₈ Minimum Maximum Mixes intercalation deintercalation Current Current Cation Other potential potential density density transferred cations (mV/SCE) (mV/SCE) 3.2 A/m² 70 A/m² Cd Zn −603 −153 100 100 Co Ni / / / / Cd Ni −582 −200 100 100 Zn Mn −741 −345  93  98 Cd Co −647 −180 100 100 Co Fe / / / / Ni Fe / / / / Cd Co/Ni −656 −166 100 100 In Cd/Zn −540 −340  52  60

It can be seen that the selectivity rate depends on the current density. The selectivity is high for high current densities thus inducing high transfer rates. The nature of the active layer 22 of the wall plays an important role in the selective transfer of cations. The transfer selectivity of Cd²⁺ or of Zn²⁺ in the presence of Ni²⁺ is truly improved up to 99% using a selenium matrix. The same observation can be made for the case of Cd²⁺ in the Cd/Co mix. The selectivity is not affected by the low thickness of the active layer 22.

Example 2

In this example, the active layer 22 is made with material LiCoO₂ with a thickness of around 80 μm. Such a material is used for example in the positive electrode of lithium ion batteries. The first electrolyte is a 1 M aqueous solution of Li⁺ in 1M Na₂SO₄ medium. The second electrolyte which acts as valorisation solution is a 1M aqueous solution of Na₂SO₄. The results are given in table 7.

TABLE 7 Current density 4 8 12 16 32 41 (A/m²) Transfer faradaic 98 99 98 99 99 41 yield (%) Transfer rate 0.14 0.2 0.41 0.6 1.21 0.82 (mol/h/m²)

Example 3

This example is similar to example 2, except that the second electrolyte which acts as valorisation solution is a solution of a propylene carbonate solvent and perchlorate ammonium tetrabutyl. The anode is made of platinum-coated titanium and the cathode is made of stainless steel. The results are given in table 8.

TABLE 8 LiCoO₂ on porous support at 10 A/m² Transfer faradaic yield (%) 75 Transfer rate (mol/h/m²) 0.401

Example 4

From a film produced by depositing Mo₆S₈-PVDF on a porous disc according to the protocol described above, the excess material of the film is eliminated by manual polishing, by abrasion with SiC disc with a grain size of 2400 for several seconds until the colouring of the porous support appears. Thus, the part of the transfer materials is left only in the pores. This operation done and tested in the same manner as the previous experimentations offers clearly more advantageous transfer performances with an applicable current density of around 80 A/m², higher than previously, without disturbing the faradaic yields.

Example 5: transfer of Li+ from a mixed Li₂SO₄ and CoSO₄ electrolyte

In this example, the active layer is made with material LiMn₂O₄ with a thickness of around 80 μm. The first electrolyte is an aqueous solution containing cations Li⁺ (1M) and Co²⁺ (0.5M) in sulphate medium. The second electrolyte which acts as valorisation solution is an Na₂SO₄ solution with a concentration of 1M. The results are given in table 9.

TABLE 9 LiMn₂O₄ on porous support Current density (A/m²) 40 70 90 110 Lithium transfer faradaic 94 96 97 98 yield (%) Lithium transfer rate 1.43 2.24 3.06 3.67 (mol/h/m2) 

1-19. (canceled)
 20. Electrolyte separation wall including a sealed active layer of a material capable of developing intercalation and deintercalation reactions for selective transfer of cations through the wall, and a support layer consisting of a porous material acting as support for the active layer.
 21. The wall according to claim 20, wherein the porous material is selected from the group consisting of mullite, silica, glass fibre, quartz and a ceramic.
 22. The wall according to claim 20, wherein porosity of the porous material is between 0.4 and 0.6.
 23. The wall according to claim 20, wherein the material of the active layer is a binary or ternary material behaving as host network and with cation reversible accommodation properties according to an oxidation-reduction reaction.
 24. The wall according to claim 23, wherein the material of the active layer is a metallic chalcogenide.
 25. The wall according to claim 24, wherein the metallic chalcogenide is a chalcogenide with molybdenum clusters (Mo_(n)X_(n+2) or M_(x)MO_(n)X_(n+2)) .
 26. The wall according to claim 23, wherein the material of the active layer is a compound of lithium and a metal in oxide, phosphate or fluoride form or a combination of these forms, and the metal being selected from the group consisting of nickel, cobalt, iron, manganese, vanadium, titanium and chrome.
 27. A manufacturing process for an electrolyte separation wall comprising: preparing a solution including an active material in the form of a powder, a binder and a solvent, coating a surface with a support layer of porous material with said solution, and evaporating the solvent to form a sealed active layer on the support layer.
 28. The process according to claim 27, comprising using poly(vinylidene fluoride) as said binder.
 29. The process according to claim 27, comprising using 1-methyl-2-pyrrolidone as the solvent.
 30. The process according to claim 27, comprising providing the material in powder form in a proportion of 80% in weight excluding the solvent.
 31. The process according to claim 27, comprising providing the material in powder form with a grain size between 30 and 100 μm.
 32. A process according to claim 27, wherein the solution includes graphite in powder form.
 33. The process according to claim 27, comprising polishing the active layer until the support layer appears through the active layer.
 34. A cation selective extraction process by electrochemical transfer wherein a transfer wall is used as an electrolyte separation wall and a transfer of cations is ensured through said transfer wall by generating a potential difference (ΔE) between a first electrolyte and a second electrolyte or said transfer wall to induce an intercalation of cations in the transfer wall on a side of the first electrolyte, and a scattering of the cations in said transfer wall then deintercalation of the cations in the second electrolyte.
 35. The process according to claim 34, comprising using a non-aqueous electrolyte for at least one of the electrolytes.
 36. The process according to claim 34, comprising electrically connecting the transfer wall to a device measuring a potential between said wall and reference electrodes located respectively in each of said electrolytes and adjusting potential applied to said electrolytes.
 37. The process according to claim 34, comprising generating wherein the potential difference (ΔE) between the first electrolyte and the transfer wall and the step of deintercalation of the cations on the side of the second electrolyte comprising performing a chemical deintercalation by a chemical oxidising agent in the second electrolyte.
 38. The process according to claim 34, comprising ensuring a succession of cation transfers through transfer walls arranged successively between said electrolytes and at least one intermediary electrolyte between the transfer walls. 